The invention is based on an illumination unit having at least one LED as light source, in accordance with the preamble of claim 1. It is in particular an LED which emits in the visible or white region and is based on an LED which emits primarily UV.
An illumination unit having at least one LED as light source, which emits, for example, white light, is currently obtained predominantly by combining a Ga(In)N-LED, which emits in the blue at approximately 460 nm, and a yellow-emitting YAG:Ce3+ phosphor (U.S. Pat. No. 5,998,925 and WO 98/12757). For good color rendering, two different yellow phosphors are often used, as described in WO 01/08453. A problem in this case is that the two phosphors often have different temperature characteristics, even if their structures are similar. A known example is the yellow-luminescent Ce-doped Y garnet (YAG:Ce) and the (Y,Gd) garnet which, by comparison, is luminescent at a longer wavelength. This leads to fluctuations in the color locus and changes in the color rendering at different operating temperatures.
The publication xe2x80x9cOn new rare-earth doped Mxe2x80x94Sixe2x80x94Alxe2x80x94Oxe2x80x94N materialsxe2x80x9d by van Krevel, TU Eindhoven 2000, ISBN 90-386-2711-4, Chapter 11 has disclosed a class of phosphor materials which are known as sialons (xcex1-sialons), which represents a contraction of their structure, which sialons may be doped with Ce, Eu or Tb. An emission in the range from 515 to 540 nm with excitation at 365 nm or 254 nm is achieved by means of doping with Ce.
It is an object of the present invention to provide an illumination unit having at least one LED as light source, the LED emitting primary radiation in the range from 300 to 430 nm, this radiation being completely converted into longer-wavelength radiation by phosphors which are exposed to the primary radiation of the LED, which is distinguished by a high level of constancy at fluctuating operating temperatures. A further object is to provide an illumination unit which emits white light and in particular has a good color rendering and a high output. A preferred primary radiation of the LED is 380 to 420 nm.
This object is achieved by the following features: the conversion takes place at least with the aid of a phosphor which emits green with a peak emission wavelength at 495 to 540 nm and which originates from the class of the Ce-activation sialons, the sialon corresponding to the formula Mp/2Si12xe2x88x92pxe2x88x92qAlp+qOqN16xe2x88x92q:Ce3+, where M=Ca individually or in combination with Sr, with q=0 to 2.5 and p=1.5 to 3. Particularly advantageous configurations are given in the dependent claims.
According to the invention, the phosphor used for the LED-based illumination unit is a sialon which emits green and originates from the class of the Ce-activated sialons, the sialon corresponding to the formula Mp/2Si12xe2x88x92pxe2x88x92qAlp+qOqN16xe2x88x92q:Ce3+, where M=Ca individually or in combination with Sr, where q=0 to 2.5 and p=1.5 to 3. It is preferable to select a high value for p, specifically p=2.5 to 3, and a relatively low value for q, specifically q=0 to 1, in particular up to 0.8. It is preferable for Ca alone to be used for the cation M.
The Ce content, which replaces some of the cation M, should be 0.5 to 15%, preferably 1 to 10%, in particular 2 to 6%, of the M cation, so that the emission wavelength can be selected particularly accurately and the light efficiency can also be optimized. An increasing Ce content generally shifts the peak emission toward longer wavelengths.
Particular advantages of this phosphor in connection with an LED-based illumination unit are its high efficiency, its excellent thermal stability (no sensitivity to changes in the operating temperature) and a surprisingly high luminescence extinction temperature, as well as the high color rendering which can be achieved thereby, in particular in combination with at least one further phosphor. The extinction temperature, i.e. the temperature at which the luminescence is destroyed on account of the heat supplied, is even so high as to lie outside the preselected measurement range (maximum 140xc2x0 C.).
A further advantage of this class of phosphors is that the starting material (in particular Si3N4) is already present in extremely finely dispersed form. Consequently, this phosphor no longer has to be milled, which eliminates one operation without any losses in efficiency. Typical mean grain sizes of the phosphor are 0.5 to 5 xcexcm. By contrast, conventional phosphors, such as YAG:Ce, have to be milled, so that they remain dispersed in the casting resin and do not sink to the bottom. This milling operation often leads to loss of efficiency. Despite having a fine grain size of the starting material, the phosphor according to the invention has good absorption.
In addition to the production of a colored light source by excitation by means of UV radiation from an LED, in particular the generation of white light with the aid of this phosphor offers advantages. This is achieved with an UV-emitting LED as primary light source by using at least three phosphors.
White light with good color rendering is achieved in particular by the combination of an UV LED (primary emission at 300 to 430 nm, preferably 380 to 420 nm), a green phosphor according to the invention (emission between 495-540 nm) and a blue-(emission: 440-480 nm) and a red-emitting phosphor (emission: 560-620 nm).
The green phosphor used is Mp/2Si12xe2x88x92pxe2x88x92qAlp+qOqN16xe2x88x92q:Ce3+. In this formula, M=Ca individually or in combination with Sr, and the Sr content is preferably less than 30 mol %. This green phosphor has an excellent thermal stability and very good luminescence at relatively high temperatures which are typical of LEDs: up to 80xc2x0 C., it does not show any drop in luminescence within the scope of the measurement accuracy. By contrast, the conventional garnet phosphors have a significantly measurable drop in luminescence at 80xc2x0 C.: this drop may be 5 to 30%, depending on the selected cation composition in the system (Y,Gd,Lu)3(Al,Ga)5O12.
The major advantages of Ce sialons are their excellent stability with respect to hot acids, lyes and also their thermal and mechanical stability. Surprisingly, these sialons have an excellent temperature extinction of the luminescence from activator ions in these compounds. This makes these compounds strong competitors for phosphors which sometimes illuminate more brightly and/or efficiently at room temperature but in use have a luminescence loss through temperature extinction. For example, Sr4Al14O25:Eu2+, which illuminates blue-green, has a quantum efficiency of approximately 85% at room temperature. At 100xc2x0 C., however, the efficiency has fallen to approximately 60%. Depending on its cation composition (MGa2S4:Eu2+), thiogallates doped with Eu2+ may luminesce throughout the entire green region but likewise lose 20%-30%. Yellow-luminescing Ce3+-doped garnets lose approximately 10-30% of the room temperature efficiency at elevated temperature, depending on the proportion of Gd:Y and Al:Ga.
A white mixture can also be produced on the basis of an UV-emitting LED by means of these Ce-doped sialons together with a blue phosphor, such as for example BaMgAl10O17:Eu2+ (BAM), Ba5SiO4(Cl, Br)6:Eu2+, CaLa2S4:Ce3+ or (Ca,Sr,Ba)5(PO4)3Cl:Eu2+ (SCAP). A further constituent is a red phosphor, such as (Y,La,Gd,Lu)2O2S:Eu3+, SrS:Eu2+ or Sr2Si5N8:Eu2+.
If necessary, the color rendering can be improved still further by adding a further green phosphor with a shifted emission maximum (for example Eu-doped thiogallates or Sr aluminates). A further possibility is for the Ce-doped sialon to be used as the only phosphor for achieving an LED with colored emission.
Depending on the Ce3+ content, the body color of this material, in particular at a low oxygen content, is almost white through pale green to dark green. On account of the excellent temperature stability and also mechanical stability, this Ce sialon is eminently suitable for use as an environmentally friendly green pigment or phosphor for a very wide range of applications. This applies in particular if M is replaced by 5 to 10% Ce.
In general, the lower q and the higher p are selected to be, the higher the quantum efficiency becomes. A phosphor with M=Ca, p=3 and q=0 is particularly preferred.
The light which the phosphor according to the invention emits under UV light is highly unsaturated green, with color coordinates of xxcx9c0.22/yxcx9c0.41. However, the luminescence is dependent on the Ce3+ content: the emission shifts toward longer wavelengths as the Ce3+ content rises.
Surprisingly, the optical properties differ from those of the sialons described in the literature, at least with a low oxygen content (below 5 mol % of the nitrogen content, preferably below 2 mol %) and a high cation content (p=1.5 to 3). The samples are often white to greenish white and therefore do not absorb in the blue region and luminesce at considerably shorter wavelengths than those described in the literature. This means that the phosphor according to the invention is very eminently suitable for UV LEDs (or if appropriate UV applications), and that in particular it does not have any competing absorption of blue light, so that a relatively long-wave UV primary emission (380 to 420 nm) can be selected. The longer the wavelength which it is possible to select for the UV emission, the lower the amount of energy consumed becomes and the more gentle the process is on the LED, which extends the surface life. In particular phosphors which contain little or no oxygen, i.e. up to at most 5 mol %, based on nitrogen, corresponding to the formula Mp/2Si12xe2x88x92pxe2x88x92qAlp+qOqN16xe2x88x92q:Ce3+, where M=Ca individually or in combination with Sr, in particular where q=0 to 0.7 and p=2.5 to 3, have these advantageous characteristics.